gstinfo: Add gst_debug_log_literal() function

This takes a plain message string and not a format string, and as a
result doesn't have to be passed through vasprintf() and lead to further
unnecessary allocations. It can also contain literal `%` because of

The new function is mostly useful for bindings that would have to pass a
full string to GStreamer anyway and would do formatting themselves with
language-specific functionality.

Part-of: <gstreamer/gstreamer!1356>
25 jobs for !1356 with gst_debug_log_literal in 55 minutes and 48 seconds (queued for 14 seconds)
merge request